Summary: Geophys. Astrophys. Fluid Dynamics, 2002, Vol. 96, No. 1, pp. 31­48
EXPERIMENTS ON INSTABILITY OF COLUMNAR
VORTEX PAIRS IN ROTATING FLUID
YA. D. AFANASYEV*
Department of Physics and Physical Oceanography,
Memorial University of Newfoundland,
St. John's, NF, Canada A1B 3X7
(Received 16 February 2001; Accepted on 10 September 2001)
We present results from a new series of experiments on the geophysically important issue of the instability of
anticyclonic columnar vortices in a rotating fluid in circumstances such that the Rossby number exceeds
unity. The vortex pair consisting of a cyclonic and an anticyclonic vortex is induced by a rotating flap in
a fluid which is itself initially in a state of solid-body rotation. The anticyclonic vortex is then subject to
either centrifugal or elliptical instability, depending on whether its initial ellipticity is small or large, while
the cyclone always remains stable. The experimental results demonstrate that the perturbations due to cen-
trifugal instability have a typical form of toroidal vortices of alternating sign (rib vortices). The perturbations
due to elliptical instability are of the form of sinuous deformation of the vortex filament in the plane of max-
imal stretching which corresponds to the plane of symmetry for the vortex pair. The initial perturbations in
both cases are characterized by a definite wave number in the vertical direction. The characteristics of the
unstable anticyclone are determined by the main nondimensional parameter of the flow ­ the Rossby
number. The appearance of both centrifugal and elliptical instabilities are in accord with the predictions